Field of the Invention
The present invention relates to a magnetic toner to be used in electrophotography, an image forming method for visualizing an electrostatic image, and a toner jet (hereinafter sometimes referred to simply as “toner”).
Description of the Related Art
In recent years, a copying machine, a printer, or the like has started to be required to have a higher speed and a longer lifetime, and hence a magnetic toner has started to need to be capable of standing longer use than a related-art one. Further, there has been a growing demand for the energy savings of an apparatus, and at the same time, excellent low-temperature fixing performance of the toner has been strongly required for corresponding to the demand.
In general, the low-temperature fixing performance is related to the viscosity of the toner, and hence the property by virtue of which the toner quickly melts with heat at the time of fixation, i.e., the so-called sharp melt property has been required.
As described in Japanese Patent Application Laid-Open No. 2004-138920, there has been proposed a toner containing toner particles each improved in sharp melt property through the incorporation of a crystalline block polyester, in which the surface coverage of each of the toner particles with an external additive is set to as high as 100% or more. Japanese Patent Application Laid-Open No. 2004-138920 proposes that the development stability of the toner be improved by such procedure while its low-temperature fixability is achieved. However, when it is assumed that the copying machine, the printer, or the like has a higher speed and a longer lifetime in the future, it is expected that an external stress such as stirring in its developing unit or an increase in temperature of its main body further strengthens, and hence a reduction in developability, an image defect, or melt adhesion to members occurs owing to the embedding of the external additive. Accordingly, the toner is susceptible to improvement.
With a view to suppressing such embedding, many attempts each involving using an external additive having a large particle diameter have been made to suppress the embedding of the external additive in the surface of the toner and to improve its development durability.
As described in, for example, Japanese Patent Application Laid-Open No. 2002-318467, Japanese Patent Application Laid-Open No. 2005-202131, and Japanese Patent Application Laid-Open No. 2013-92748, it has been proposed that spacer particles be added to suppress the embedding of the external additive and to improve the endurance stability of the toner. However, the addition of those spacer particles is expected to adversely affect the low-temperature fixability of the toner.
Further, it has been known that inorganic fine particles, or organic-inorganic composite fine particles each using a resin having a high crosslinking density as a core resin, to be generally utilized as the spacer particles have a high volumetric specific heat. Accordingly, when a quantity of heat by which the temperature of the external additive can be sufficiently increased is charged into a fixing unit, there is a risk in that the temperature of a toner image after fixation hardly reduces, and hence the phenomenon in which upon lamination of sheets of paper immediately after printing, the sheets of paper adhere to each other, i.e., the so-called adhesion of printed paper occurs.
As described above, in consideration of increases in speed and lifetime of a printer or the like, and the energy savings thereof in the future, a toner having high developability, and excellent in low-temperature fixability and resistance to the adhesion of printed paper is needed. At present, however, there are an extremely large number of technological problems to be solved for such purpose, and the related-art toner is susceptible to improvement.